Method of producing germanium pellets



May 6, 1952 w. c. DUNLAP, JR

METHOD OF PRODUCING GERMANIUM PELLETS Filed Dec. 25, 1949 RJTI,

m WM ii a z m S Patented May 6, 1952 METHOD QF PRODUCING GER-MANIUMPELLETS William C. Dunlap, Jr., Schenectady, N. Y., assignor to GeneralElectric Company, a corporation of New York Application December 23,1949', Serial No. 134,761

2 Claims.

My invention relates to methods for producing metallic pellets and moreparticularly to shottower methods for producing germanium pelletssuitable for use in asymmetrically conductive devices, such as describedand claimed in application Serial No. 134,826 of Harper Q. North, filedconcurrently with the present application and assigned to the sameassignee.

The above-mentioned North application discloses that tiny pellets ofproperly prepared germanium are suitable for use in asymmetricallyconductive devices. Accordingly, a principal object of my invention isto provide an improved shot-tower method for quickly and prolificallyproducing such small germanium pellets of substantially uniformpredetermined size and composition with minimum handling or care.

Another more specific object of my invention is to providean improvedshot-tower method for converting an ingot of highly purified germaniuminto tiny pellets with little danger of chemical contamination fromundesirable impurities.

For performing my invention I provide a shottower apparatus whichenables a continuous gas pressure to be supplied across an ingot ofgermanium while it is being melted within a crucible. This gas pressureejects the germanium in the form of droplets through a small hole in thecrucible just as soon as the germanium is melted with the result thatthe molten germanium is exposed to the contaminating influence of thecrucible for only an extremely short period of time. In addition, I havefound that by properly regulating the gas pressure with reference to thesize of the crucible hole, the approximate size and rate of productionof the pellets may be controlled to a considerable extent.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims. My inventionitself, however, together with further objects and advantages thereofcan best be understood by reference to the following description takenin connection with the accompanying drawing in which the sole figure isa sectional view of a shot-tower apparatus for performing my inventionand illustrating a germanium ingot in a partially melted state.

Referring to the drawing, I have shown, apparatus for performing myinvention comprising a closed container I preferably in the form of acylindrical tube of transparent material, such as quartz or glass, atthe bottom of which a liquid bath 2 is located. Suspended within theupper end of the container I from a lid or cover 3 of thecontainer I isa cylindrical hollow rod 4, preferably of graphite, which constitutes agas conduit. Detachably secured to the bottom of rod 4 is a crucible 5in which an ingot of germanium 8 may be placed. The crucible 5 iscomposed of a material, such as graphite which does not readily combinewith molten germanium and has a small hole or aperture 1 at the bottomthereof. The sides of the crucible 5 leading to hole 'I are preferablytapered, as illustrated, and the size of the hole I, as will be morefully explained hereinafter, determines to a great extent the size ofthe pellets that will be produced. Crucible holes in the neighborhood of.020 inch in diameter have been found to give excellent results althoughconsiderable variation of the diameter may be the crucible and functionsin a well-known maner to melt the germanium ingot within the crucible bythe application of a high frequency magnetic field therethrough.

Means are also provided whereby the entire system may be flushed with achemically inactive gas, and the molten germanium within the cruciblemay be subjected to a controlled gas pressure to force the germanium outof the crucible'hole I in the formof droplets comprising a stream orspray. A chemically inactive gas which may, for example, be dry nitrogenor helium which does not readily combine with molten germanium is fedinto the container i through a gas inlet 9 containing a pressurecontrolling valve l0 and extending through the lid 3 to communicate withthe gas conduit graphite rod 4. The crucible 5 is preferably secured tothe gas conduit 4 such as by threading, as illustrated, so that theconduit completely covers the mouth of the crucible leaving the cruciblehole I as the only outlet into container. Gas outlet valves I l arepreferably provided in the top and bottom of the container l in order toallow the entire container l to be flushed out by the pressure of theentering gas. It will be readily appreciated that the gas enteringthrough inlet 9 will pass through the gas conduit 4 and through thecrucible hole 1 into the container I in order to enable this flushingaction.

In the operation of my invention, the prepared ingot 6 of highlypurified germanium is placed within the crucible and the entirecontainer flushed with a chemically inactive gas as described above. Ahigh frequency electrical current of suflicient magnitude to melt thegermanium within the crucible 5 is passed through the coil 8, and thegas pressure is regulated by ad- J'ustment of the inlet and outletvalves to a proper value as more fully explained below. As the germaniummelts it runs down the sides of the crucible 5 and is forced out thehole 1 in the form of droplets I2 by the pressure of the applied gas.The droplets l2 are forced out of the crucible hole 1 in the form of astream or spray and fall through the previously established chemicallyinactive atmosphere within the con tainer l into the liquid bath 2below. If the bath 2 is one foot or more below the crucible 5, thedroplets [2 cool sufliciently as they fall so that they are not normallycontaminated by the liquid bath 2 although distilled water is preferablyemployed as the bath in order to insure against such contamination. Thedroplets I2 are quickly quenched by the bath 2 and solidify into tinypellets I3 of germanium, as indicated at the bottom of the container I.

It is evident that because of the small size of crucible hole 1, theforce of gravity alone is insufficient to overcome the surface tensionof the molten germanium and to pull the germanium through the hole. Foreach size hole there is a certain threshold pressure which barelyovercomes the surface tension in order to force the germanium throughthe hole. This threshold gas pressure ranges from a pressure ofapproximately 9 centimeters of mercury for a hole of .015 inch to apressure of approximately 5 centimeters of mercury for a hole of .040inch. I have found, however, that upon the application of a gas pressureimmediately above this threshold pressure, the germanium droplet whichis forced through the hole 1 tends to hang momentarily at the orificeand to grow slightly larger before it drops off, with the result thatpellets are produced of variable diameter considerably larger than thediameter of the hole. As the pressure is further increased the dropletsare blown throu h the hole more rapidly and the diameter of theresultant pellet becomes smaller and more uniform as it approaches thediameter of the hole. Once the size of the droplet becomes comparablewith the size of the hole, any further increase of gas pressure merelyincreases the rate of production. It will, of course, be appreciatedthat the rate of production of these pellets is further limited by thespeed of melting of the germanium ingot. With a strong heating 4 currentcausing rapid melting of the ingot and a high gas pressure, over 20,000pellets of relatively uncontaminated germanium have been produced bythis method within a few minutes after the germanium ingot begins tomelt.

It is to be understood that while I have described a particularembodiment of my invention, many modifications can be made, and I,therefore, intend by the appended claims to cover all such modificationsas fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. The method of producing germanium pellets, which method comprisesplacing a germanium ingot within a crucible having a small hole at thebottom, passing under predetermined pressure a gas chemically inactiveto germanium around the ingot and through the crucible hole, and thenheating the germanium ingot above its melting point while it issubjected to the influence of the encompassing gas to form moltendroplets of germanium blown out of the hole by the force of said gaswhile the germanium is melting,

2. The method of producing germanium pellets suitable for use inasymmetrically conductive devices, which method comprises placing aningot of germanium within a crucible having a small hole at the bottom,forcing under predetermined pressure a flow of gas chemically inactiveto germanium around the germanium and through the crucible hole, andthen passing through the ingot a high frequency magnetic field ofsufiicient intensity to melt the ingot while the ingot is subjected tothe encompassing gas flow, said gas flow forcing germanium through thehole while it is being melted to form germanium droplets having littlechemical contamination from the crucible.

WILLIAM C. DUNLAP, JR.

REFERENCES CITED The following references are of record in the

1. THE METHOD OF PRODUCING GERMANIUM PELLETS, WHICH METHODS COMPRISESPLACING A GERMANIUM INGOT WITHIN A CRUCIBLE HAVING A SMALL HOLE AT THEBOTTOM, PASSING UNDER PREDETERMINED PRESSURE A GAS CHEMICALLY INACTIVETO GERMANIUM AROUND THE INGOT AND THROUGH THE CRUCIBLE HOLE, AND THENHEATING THE GERMANIUM INGOT ABOVE ITS MELTING POINT WHILE IT ISSUBJECTED TO THE INFLUENCE OF THE ENCOMPASSING GAS TO FORM MOLTENDROPLETS OF GERMANIUM BLOWN OUT OF THE HOLE BY THE FORCE OF SAID GASWHILE THE GERMANIUM IS MELTING.